A system is proposed that combines three different techniques to assess the cognitive functions of patients with excessive daytime sleepiness. The patient's state is assessed by Event Related Potentials, behavioral measures from a vigilance test, and polysomnography. ERP recordings during an auditory oddball task provide objective measures of the speed of cognitive processing and the patient's degree of attentional/motivational involvement in the task. Timing button press responses to target tones provide a behavioral measure of processing speed, including motor as well as cognitive components. Omission and commission response errors index attention lapses. Vigilance decrement is assessed by comparing the first and second halves of the session. EEG measures, such as alpha and theta power, will further index patient state over the course of the task. In addition to sleep laboratory applications, a portable version of the device will provide testing capabilities at the home of the patient. For OSA patients, these tests can be administered by home care technicians visiting the patient on CPAP therapy to follow up progress, monitor compliance, and adjust equipment parameters. This new diagnostic tool will be used to follow the effect of treatment on cognitive impairment in sleep disorders. PROPOSED COMMERCIAL APPLICATIONS: This system will provide a new, innovative and effective diagnostic tool for the assessment of cognitive states of patients with sleep disorders, in the laboratory and at home. It will be offered in a portable version as a stand- alone unit and as an option on digital polysomnography and Evoked Potential systems. It will be marketed to sleep laboratories for clinical use, and also for research applications on Excessive Daytime Sleepiness and/or Event Related Potentials.